The invention relates to apparatus for metal refining and is directed to an improved molten metal charge well for melting solid metal pieces into a molten metal bath. The invention is particularly applicable to aluminum refining.
In the secondary or scrap metal industry, metal refining apparatus typically include a large furnace which provides molten metal to a charge well, where scrap metal pieces or particles are added to the molten metal. A molten metal pump is provided for conveying the molten metal from the furnace to the charge well. From the charge well, the metal flows into a skim well where impurities are removed prior to reintroduction of the metal to the furnace. The molten metal in the charge well typically includes a skim and dross layer on top which insulates the molten metal from the atmosphere. In this way, scrap metal may be melted while the generation of metal oxides, which are detrimental to the refining process, is prevented or reduced.
With lightweight metals such as aluminum scrap metal, which have a high surface area to weight ratio, there is a tendency for the metal particles to float on top of the molten metal in the charge well. Moreover, the molten metal has a high surface tension which results in large buoyant forces. Floating scrap metal is undesirable because it remains exposed to the atmosphere for a sufficient period of time while being heated by the underlying molten metal that the formation of oxides and the introduction of other impurities often results.
It is known to provide a rotatable impeller within the charge well to facilitate the submergence of the incoming scrap metal pieces. Such devices are disclosed, for example, in U.S. Pat. Nos. 4,598,899 and 4,322,245, which are incorporated herein by reference. One problem with such devices, however, is that the impeller motion often results in the formation of a vortex that is sufficient to draw surrounding atmospheric gases into the molten metal. Such devices are often complex and costly to manufacture and maintain and their maintenance may result in significant down time.
It would therefore be desirable to provide a device for facilitating the introduction of scrap metal into the molten metal bath in the charge well without the attendant disadvantages of the prior art devices. Accordingly, it is an object of the invention to provide a charge well configuration which permits the facile and efficient introduction of scrap metal pieces into the molten metal bath with a minimization of melt loss.
A variety of charge well constructions were evaluated by the applicants. Four vortex wells were water modeled at a 5/6 scale in the laboratory. The key desirable parameters of the designs include: a) facile submergence for quick melting (heat transfer) and high feed rates; and b) a relatively calm surface of the molten metal bath for minimization of dross formations in a molten aluminum application. In Test 1, molten metal was directed tangentially into a cylindrical charge well and removed in a tangential direction to establish a vortex. In Test 2, molten metal was directed tangentially into a bowl-shaped charge well via a somewhat restrictive conduit which established a relatively high pressure flow. In this case the molten metal entered near the top of the charge well and was withdrawn at the base. With reference to Test 3, molten metal was provided to the charge well via an upwardly sloped relatively wide diameter conduit which did not provide a pressure increase of the magnitude achieved by the conduit in Test 2. Again, the molten metal was withdrawn from the base of the charge well. Test 4 used a molten metal conduit similar to that utilized in Test 3. However, a flow diverter was inserted into the charge well to achieve a downward flow of molten metal within the generally circular flow created by the tangential inlet.
As the results of the evaluations indicate, relative to the other tests, the pressure of a downward flow diverter decreased the time required to submerge 50 plastic sticks and expel 90% of them relative to the other tests. It is significant to note that this improvement over even Tests 1 and 2 occurred with an inlet ramp and outlet design resembling that of Test 3 which was unable to discharge any sticks.
The flow diverter of Test 4 was comprised of a wedge-shaped body having an arcuate downward facing surface directing flow downward. The body also includes a relatively wide leading edge which tapers to a narrow trailing edge.